1. Field of the Invention
The present invention relates to a sintered sprocket to be used as a sprocket of a cam shaft timing chain of an automotive engine and to a production method therefor.
2. Related Art
As such a sprocket, those formed into sprocket members by forging steel materials, finished by machining, and case hardened and tempered are known. As a sintered sprocket to be produced by a powder metallurgy technique by which forming is facilitated, techniques of producing sprockets made of ferrous materials containing Ni, Mo, Cu, and C by warm-compacting a powder mixture to a relatively high density of around 7.3 Mg/m3 and either sintering formed bodies at a temperature of 1,180° C. or higher or case hardening or induction hardening the sintered bodies; and then tempering the obtained bodies have been proposed (see Japanese Patent Application Laid-Open No. 2001-295915, page 2; hereinafter referred to as Patent Document No. 1). Furthermore, techniques involving densifying the gear teeth of sintered bodies made of ferrous materials with similar compositions by forming by rolling and then case hardening and tempering the gear teeth have been proposed (reference to Japanese Patent Application Laid-Open No. 2002-129295, page 2; hereinafter referred to as Patent Document No. 2).
Since a sprocket for a silent chain has a structure of interlocking gears and transmits rotation by interlocking the gears, as compared with that of a common sprocket for a roller chain, the interlocking contact pressure of the sprocket for a silent chain is high. Therefore, the sprocket for a silent chain is required to have a high density in the gears as compared with that of a sprocket for a roller chain and also a high hardness by hardening.
The sintered sprockets described in the above-mentioned Patent Documents Nos. 1 and 2 are, respectively, superior; however, as described in the Patent Document No. 1, in the case in which the entire density is adjusted to be relatively high, at about 7.3 Mg/m3, by employing warm-compaction for powder compacting and sintering is performed at a temperature as high as 1,180° C. or higher, the sintering temperature is high, and therefore the production cost is adversely increased and the overall density becomes high due to the warm-compaction. However, since the density of the gear faces and the density of other parts are the same, there is still some room remaining for further improvement of the contact pressure resistance of the gear faces.
Also, as described in the Patent Document No. 2, in the case in which the gear teeth of the sintered body of a desirable alloy structure with an alloy composition similar to those described in the Patent Document No. 1 and containing a ferrite structure of 40% or more by surface area and having good plastic processibility are densified by forming by rolling, the surface is further densified, and therefore, the forming by rolling margin must be made wide, and consequently, there is still some room remaining for further improvement in the production efficiency.